It is proposed to continue the analysis of genetic interactions in a bacteriophage system consisting of a satellite virus (P4) and its helper (phage P2 or P2 relatives). The main aim is to elucidate the genetic control mechanisms that couple the expression of satellite and helper genomes. Specifically it is proposed to investigate further: (1) the mechanisms (derepression and alloactivation) by which P4 activates helper gene expression, by: (a) continuing the exploration of the role of the alpha subunit of the bacterial RNA polymerase (gene rpoA product) in the expression of P2 late genes, using P2 and P4 mutants (ogr) able to grow in an rpoA mutant (gro) nonpermissive for wild type phage; (b) studying bacterial strains (other than gro), P2 and P4 strains carrying mutations that interfere with P2 gene activation by P4 and isolating additional mutants for such studies; (c) studying the "helper incompetence" of certain P2 related phages; (2) the derepression of P4 lysogens by P2, in particular the role of the P2 cox function(s) in derepression; (3) the interactions between P4 and the sex factor F, in particular the inhibition of certain P4 mutants by F. Further plans include: (4) a search for P2 - P4 recombinants; (5) contributions to the genetic mapping of P4. BIBLIOGRAPHIC REFERENCES: Sunshine, M. G., E. W. Six, K. Barrett and R. Calendar. 1976. Relief of P2 bacteriophage amber mutant polarity by the satellite bacteriophage P4. J. Mol. Biol. 106: 673-682. R. Calendar, J. Geisselsoder, M. G. Sunshine, E. W. Six and B. H. Lindqvist. The P2-P4 transactivation system. In: Comprehensive Virology VII, 1976 (in press).